The invention concerns a process for the production of an asymmetrical hollow filament membrane suitable for ultrafiltration and/or microfiltration, through extrusion of a spinning solution of a polyamide or a mixture of polyamides and/or copolyamides with formic acid and a coagulating core liquid into a coagulating precipitation bath liquid different from the core liquid, and subsequent drying, and asymmetrical hollow filament membranes produced thereby.
German Offenlegungsschrift DE-OS No. 26 06 244 describes a process for the production of symmetrical hollow filament membranes, whereby the spinning solution, for example composed of a 15 to 25% polyamide solution, is provided with a pore-forming substance, for example a metal salt, after which a second solvent is added, in which the pore-forming substance dissolves but which is a non-solvent for the polyamide. This spinning solution is spun through customary ring nozzles, whereby through a core nozzle a coagulating core liquid is provided, for example water or alcohol or ketone dissolved in water, into the hollow space or cavity of the spun solution. The spinning solution must then be spun into air and first enters into a precipitation bath after a determined stretch of air. The coagulating precipitation bath liquid is likewise composed of water or an alcohol or ketone dissolved in water. After the termination of coagulation, the metal salt with the second solvent must be washed out, in a second bath.
It has now been discovered that it is possible, without addition of a pore-forming substance and through the choice of the core liquid and the precipitation bath liquid, to produce asymmetrical hollow filament membranes of polyamide. Due to the selection of a particular pore-forming substance, the second washing bath for washing out the pore-forming substance is superfluous. Through suitable choice of the coagulating core and precipitation bath liquids the pore gradient of the hollow filament membrane can be adjusted as desired.